The background description provided herein is for the purpose of generally presenting the context of the invention. The subject matter discussed in the background of the invention section should not be assumed to be prior art merely as a result of its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions. Work of the presently named inventors, to the extent it is described in the background of the invention section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the invention.
Among neural prostheses, cochlear implants (CI) are considered the most successful devices. To date, they restore some hearing in about 324,200 severe-to-profound deaf individuals by stimulating segments along the length of the tonotopically organized cochlear spiral ganglion[25]. The average cochlear implant user can understand running speech under quiet listening conditions. The scores in standard speech recognition tests for many of the patients are more than 50% and can reach close to 100%. However, the performance suffers in noisy listening environments and for music perception. There is a need in the art for improved neural prostheses that provide improved sensory experiences for patients with impaired hearing.
Auditory stimuli are processed through a complex set of intricate anatomical structures and cellular processes that convert the acoustical stimuli into electrical signals that are then processed into sensory perceptions. The challenges in designing neural prostheses are best understood with a detailed description of the structures and processes involved in, the transmission of auditory stimuli to the brain.
Sound waves travel along the outer ear canal and vibrate the tympanic membrane, which separates the outer ear canal from the middle ear. Vibrations of the tympanic membrane are transmitted through the middle ear to the cochlea by a chain of three ossicles (malleus, incus and stapes). The middle ear compensates some of the impedance-mismatch that exists between the air and the fluid filled space of the cochlea. The stapes inserts into the oval window, an opening at the basal turn of the cochlea. Vibrations of the stapes result in pressure changes in the fluid filled space of the inner ear. A second window in the cochlea, the round window, allows displacing fluids caused by the pressure changes, and vibrations of cochlear soft tissue structures resulted.
Tissue properties, such a stiffness of the soft tissue, change along the cochlea being stiffer at the cochlear base and increasingly compliant towards the cochlear apex. Thus, frequency and level of an acoustic stimulus determine amplitude of the soft tissue vibration and the location site of the vibration along the cochlea. High frequency acoustic stimuli vibrate the basal section of the cochlea while low frequency stimuli vibrate the cochlear apex. The cochlea acts a frequency analyzer, not only for simple pure tone stimuli but also for complex acoustical signals such as speech.
Soft tissue structures, such as the organ of Corti contain the hair cells (outer and inner hair cells) that are able to convert the mechanical vibrations of the soft tissue structures into changes of the cell's membrane potential by bending their hair like extensions, the stereocilia. Amplification of the sound induced vibrations occurs from the outer hair cells, which change their length upon stimulation and the detection of the vibration and subsequent transmitter release by the inner hair cells.
In severe-to-profound deaf, most of the hair cells have been lost and the acoustical signals cannot be transmitted to the remaining auditory nerve. Part of the hearing can be restored by electrically stimulating remaining auditory nerve fibers using a cochlear implant. In other words, the cochlear implant must substitute for this complex set of interactions and directly stimulate the auditory neurons, effectively mimicking the tonotopic arrangement. Commercially available implants stimulate the neurons electrically.
The cochlear implant comprises an electrode array that is implanted in the cochlea with contacts placed at different locations along the path from the oval window to the cochlear apex. The design of speech processors for cochlear implants relies on several assumptions regarding users' perceptual responses to electrical stimulation. These assumptions are that each electrode contact is distinct, both spatially and temporally, or equivalently that electrode interaction (i.e., overlap in fields of stimulation) does not occur. In a successful multichannel cochlear implant, stimulation at one electrode should not affect the perceptual response to stimulation resulting from neighboring electrodes.
Harmonic structure is a common feature for complex sounds, such as those produced by the human voice and musical instrument. Psychoacoustic studies show that the normal auditory system makes good use of harmonic information to perceive pitch, identify instrument timbre, and focus on a target speaker in complex listening situations. As important as it is, how to represent harmonic information in cochlear implants remains topic for research. The key difficulty lies in the inherent coarse spectral and temporal resolution of cochlear implants
The restoration of melody perception is one key remaining challenge in cochlear implants. A novel sound coding strategy is proposed that converts an input audio signal into time-varying electrically stimulating pulse trains. A sound is first split into several frequency sub-bands with a fixed filter bank or a dynamic filter bank tracking harmonics in sounds. Each sub-band signal is coherently downward shifted to a low-frequency base band. These resulting coherent envelope signals have Hermitian symmetric frequency spectrums and are thus real-valued. A peak detector or high-rate sampler of half-wave rectified coherent envelope signals in each sub-band further converts the coherent envelopes into rate-varying, interleaved pulse trains. Acoustic simulations of cochlear implants using this new technique with normal hearing listeners, showed significant improvement in melody recognition over the most common conventional stimulation approach used in cochlear implants.
Nonetheless, there remains a bottleneck for the coding for the following reasons: (1) 120 dB in acoustical level difference are compressed to a 6-12 dB change in the current level of the cochlear implant device; (2) more than 50 perceptual channels in a normal hearing subject are reduced to about 4 channels in a cochlear implant patient; and (3) the normal auditory system is characterized by its nonlinearity based on the cochlear mechanics (providing the large dynamic range of 120 dB), its sensitivity, and its frequency selectivity.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.